bpf: Emit struct bpf_xdp_sock type in vmlinux BTF

Similar to other BPF UAPI struct, force emit BTF of struct bpf_xdp_sock
so that it is defined in vmlinux.h.

In a later patch, a selftest will use vmlinux.h to get the definition of
struct bpf_xdp_sock instead of bpf.h.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925170013.1752561-1-ameryhung@gmail.com

net/core/filter.c

@@ -7439,6 +7439,8 @@ u32 bpf_xdp_sock_convert_ctx_access(enum bpf_access_type type,
 				      offsetof(struct xdp_sock, FIELD)); \
 	} while (0)
 
+	BTF_TYPE_EMIT(struct bpf_xdp_sock);
+
 	switch (si->off) {
 	case offsetof(struct bpf_xdp_sock, queue_id):
 		BPF_XDP_SOCK_GET(queue_id);